1. Field of the Invention
This invention relates broadly to surface modification of polymeric hydrogels. More particularly, the invention relates to the treatment of ophthalmic lenses, especially siloxane-containing hydrogel contact lenses.
2. Description of the Related Art
Ophthalmic lenses, such as contact lenses, are subject to a myriad of requirements. Among other things, contact lenses must be ocularly-compatible, have a high oxygen permeability, allow sufficient tear exchange, have high visual light transmission, be sufficiently thin to fit between the eye and eye-lid, and be comfortable for the wearer. Although the formation of a contact lens from one core material is desirable from a manufacturing viewpoint, many known contact lens materials have one or more disadvantages. For example, polysiloxanes are known to have high oxygen permeability, which is advantageous for a healthy cornea, but most polysiloxanes have insufficient hydrophilicity to allow adequate lens movement on the eye. In order to combine the advantages of two lens materials, research has been directed to the use of a core material which has been surface-coated to modify the core""s surface properties while retaining the bulk material properties of the core.
Generally speaking, methods of treating polymer surfaces include (1) corona discharge, (2) surface degradation or oxidation by oxidizing agents such as chromic acid, (3) plasma treatment and/or plasma polymerization, (4) graft polymerization, and (5) coating. For example, xe2x80x9cA Novel Modification of Polymer Surfaces by Photograftingxe2x80x9d (ACS Synp. Ser. v. 121, pages 217-241) discloses a method of photografting monomers to hydrophobic polymers such as polyethylene and polypropylene in a 0.2 M benzophenone initiator solution.
Another example of photografting of monomers onto polymer surfaces is disclosed in xe2x80x9cPhotochemical Grafting of Vinyl Monomers onto Starchxe2x80x9d (Starch 33 (1981) v. 3, pp. 90-97). This reference discusses the grafting of vinyl monomers onto starch under ultraviolet light excitation in the presence of a wide range of photoinitiators, including benzophenone.
U.S. Pat. No. 4,595,635, issued to Dubrow et al. on Jun. 17, 1986, discloses the surface treating of organopolysiloxane materials to increase the surface tack. One method involves applying a photoinitiator to the organopolysiloxane and irradiating the surface with ultraviolet light. Benzophenone is disclosed as a suitable photoinitiator. The treated materials are useful as tapes, sealants and encapsulants.
U.S. Pat. No. 4,892,402, issued to Sawamoto on Jan. 9, 1990, discloses a method for making hard contact lenses more hydrophilic. The method involves contacting a hard lens with a treatment solution containing a hydrophilic monomer, at least one photosensitizer selected from aromatic ketones or quinones, and a solvent, and then irradiating the lens with ultraviolet light. Thus, the disclosed method is a solvent-based polymerization. Benzophenone is an example of a suitable photosensitizer.
While the exist numerous methods of surface treating polymeric articles, there remain problems with the surface treatment of hydrogel materials, especially those suited as ophthalmic lenses. One problem with many surface grafting techniques is that monomer in the treatment solution may penetrate the hydrogel""s polymer matrix, and subsequently bind to the polymer below the article""s surface. If substantial monomer penetration occurs, the treated polymeric article may swell and distort from the original shape. Even slight distortion can produce substantial optical distortions in an ophthalmic lens. Thus, there remains a need for a method of surface treating hydrogel materials without substantially modifying the shape of the hydrogel.
An object of the invention is to modify the surface properties of a polymeric hydrogel, without substantially modifying the shape of the hydrogel.
A further object of the invention is to increase the hydrophilicity of the surface of a polymeric hydrogel, without substantially modifying the shape of the hydrogel.
Another object of the invention is to increase the hydrophilicity of the surface of a hydrophilic ophthalmic lens, without substantially impairing the optical properties of the lens.
One embodiment of the present invention is a method of modifying the surface characteristics of a polymeric article, which includes the following steps:
(a) immersing a polymeric article in a first solution including a benzophenone and at least one solvent for a predetermined time period;
(b) removing said article from said first solution;
(c) evaporating at least a portion of said solvent from said article;
(d) placing said article in a second solution containing at least one macromer; and
(e) applying radiation to said article for a period sufficient to cause at least a portion of said macromers to bond to the surface of said article,
wherein said macromer has a size sufficiently large such that no substantial amount of macromer penetrates said article and causes substantial distortion of said article.
Another embodiment is a polymeric hydrogel article including a core bulk material and a surface coating. The surface coating is formed by graft polymerization with surface photoinitiation in the presence of a benzophenone. The grafted groups are formed from one or more macromers having a size sufficiently large such that no substantial amount of macromer penetrates said lens causing substantial distortion of said lens. The preferred polymeric article is a soft contact lens including a bulk material which is a silicone-containing hydrogel.
The following terms are defined at the outset in order to facilitate a clear understanding of the present invention. Certain terms are defined subsequently in the text because of the limited use herein.
The term xe2x80x9ccopolymerxe2x80x9d, as used herein, means polymers formed from two or more monomers or macromers. Thus, copolymer includes terpolymers and the like.
The term xe2x80x9cmacromerxe2x80x9d as used herein refers to a species having at least one unsaturated bond and having at least two repeating monomer units.
The term xe2x80x9chydrogelxe2x80x9d, as used herein, refers to a polymeric material which contains at least about 10 weight percent water when fully saturated.
The present invention is a method of modifying the surface characteristics of a polymeric hydrogel. The preferred polymeric articles useful in accordance with the present invention are silicone-containing hydrogel materials. A more preferred subclass of articles which may be treated in accordance with the present invention are ophthalmic lenses, including without limitation thereto, contact lenses, intraocular lenses (i.e., implants), and lenses designed for delivery of pharmaceuticals or other agents to the ocular environment. While contact lenses are the most preferred treatment article, and the invention is discussed primarily with reference to contact lenses, the invention is not so limited.
The advantageous surface treatment method of the present invention generally involves the following steps:
(a) immersing a contact lens in a first solution including a benzophenone and at least one solvent for a predetermined time period;
(b) removing said lens from said first solution;
(c) evaporating at least a portion of said solvent from said lens;
(d) placing said lens in a second solution containing at least one macromer; and
(e) applying radiation to said lens for a period sufficient to cause at least a portion of monomers or macromers to bond to the surface of said lens,
wherein said macromer has a size sufficiently large such that no substantial amount of macromer penetrates said lens causing substantial distortion of said lens.
The term xe2x80x9cPhotosensitizerxe2x80x9d, as used herein include, without limitation thereto, benzophenone, thioxanethen-9-one, 2-acetonaphthone, mixtures thereof and the like.
The term xe2x80x9cbenzophenonexe2x80x9d, as used herein, means benzophenone and derivatives thereof which are suited to initiating surface bonding to a contact lens polymer in the presence of applied light, i.e., derivatives suited to photoinitation of surface polymerization. Thus, xe2x80x9cbenzophenonexe2x80x9d includes, without limitation thereto, 2-amino benzophenone; 2-amino-5-chloro benzophenone; 2-amino-4xe2x80x2-methyl benzophenone; 2-amino-5-methyl benzophenone; 2-amino-5-nitro benzophenone; 3-amino benzophenone; 3-amino-4-methyl benzophenone; 3-amino-4xe2x80x2-methyl benzophenone; 4-amino benzophenone; 4-amino-3-methyl benzophenone; 4-amino-4xe2x80x2-methyl benzophenone. A more preferred group of benzophenones includes benzophenone; 4-fluorobenzophenone; 4-chlorobenzophenone; 4-benzoylbenzoic acid; 3,3xe2x80x2-4,4xe2x80x2-benzophenone tetracarboxylic dianhydride, Mischler""s ketone, and mixtures thereof.
The first treatment solution is intended to functionalize the lens surface, i.e., to prepare the surface for facile grafting or bonding of macromeric species thereto. The solution may include about 0.1 to 10 millimoles/liter (mmol/L) benzophenone, about 10 to 50 weight percent solvent, and about 90 to 50 weight percent water, in which the water and solvent weight percentages are based on the water/solvent mixture. More preferably, the first solution includes about 0.5 to 5 weight percent benzophenone, about 20 to 40 weight percent solvent, and about 60 to 40 weight percent water. Suitable solvents for benzophenone include, without limitation thereto, short chain alcohols such as isopropanol, ethanol, and the like. Preferably, the solvent is miscible with water, so that a homogeneous system is produced.
While the invention is not limited to theoretical explanations of operability, the following theory is offered to better enable the reader to understand the invention. It is believed that the benzophenone separates out of the solution at unduly low solvent levels. Thus, low solvent levels may create a heterogeneous system, and promote non-uniform surface functionalization. However, increasing solvent concentration may cause the lens to swell. The swelling is believed to increase the lens porosity. At excessively high solvent concentrations, the pores in the lens material are large enough to allow penetration of the macromer into the lens structure, thereby causing lens deformation and optical distortions. Thus, a proper surface treatment is achieved by maintaining the solvent concentration high enough to provide a homogenous mixture (and uniform treatment), while maintaining the solvent concentration below the point at which unacceptable macromer penetration (and optical distortion) occurs.
The first solution may be exposed to the lens for a period of about 1 to 30 minutes, more preferably about 5 to 20 minutes, and most preferably about 8 to 12 minutes. Subsequent to contact with the first solution, the lens is removed from the first solution and evaporated under conditions which do not substantially damage the lens.
Evaporation of the solvent is preferred. Evaporation may be accomplished in a number of ways, including air drying and application of a limited amount of heat or air currents. The lens may be air dried for about 30 minutes to 3 hours, more preferably about 1 to 2 hours, depending on the temperature and relative humidity. Subsequent to air drying, the lenses may be placed in a nitrogen atmosphere. Then, a vacuum may be generated, preferably at a pressure of less than about 0.5 millibars. More preferably, the lenses are nitrogen and vacuum treated during two or more cycles.
The amount of water and/or solvent remaining in the lens depends on the specific treatment composition selected and the core polymer material chosen for treatment. It is preferable to remove the bulk of the solvent so that the photosensitizer remains on the surface of the article to be treated. Preferably, the solvent and water are substantially completely evaporated, i.e., less than 50 weight percent solvent/water remains in the lens. More preferably, less than about 25 weight percent solvent/water remains in the lens.
Preferably subsequent to the evaporation step, the lens is contacted with a second solution including macromer. The second solution preferably includes about 1 to 50 weight percent macromer and about 99 to 50 weight percent water. More preferably, the second solution includes about 5 to 10 weight percent macromer and about 95 to 90 weight percent water.
After contacting the lens with the macromer-containing solution, grafting of the macromer to the lens surface is initiated by applying ultraviolet light in an amount and for a time sufficient to cause a substantial amount of the macromer to bond to the lens surface. The UV light may be applied at an intensity of about 25 to about 100 milliwatts per square centimeter (mW/cm2) for a period of about one minute to one hour. More preferably, the UV light is applied at an intensity of about 50 to about 60 mW/cm2 for a period of about 5 to 20 minutes.
Subsequent to photoinitiation of the macromer-bonding reaction, the lens may be subjected to an optional step of extraction to remove any undesirable remaining solvent. For example, the lens may be rinsed with a solution of deionized water, followed by a solution of about 10 to 30 weight percent isopropanol with about 90 to 70 weight percent water.
A wide variety of hydrophilic macromers are useful in the present invention, including without limitation thereto, polalkylene oxides such as polyethylene oxide; polydimethylacrylamides; polyvinyl alcohols; poly(acrylates); poly(methacrylates) such as poly(2-hydroxyethyl methacrylate); poly(N-vinylpyrrolidone); poly(acrylamides) such as poly(N,N-dimethylacrylamide); derivatives, copolymers, mixtures thereof, and the like. A more preferred group of hydrophilic macromers includes polyethylene oxides, polyvinyl alcohols, and polyacrylamides, especially poly(N,N-dimethylacrylamide). Furthermore, it may be possible to apply a mixture of functionalized macromers with polymers.
However, the macromers useful in accordance with the present invention must have a size sufficient to prevent the macromer from substantially penetrating the polymer network of the lens. If the polymer network is penetrated, the lens may swell and become distorted. Minor amounts of swelling can cause intolerable amounts of deviation in the intended overall or localized optical power of the lens, thereby distorting the vision of the ultimate consumer. Also, a poor fit to the consumer""s eye can occur if the lens swells. Moreover, the unpredictability of the swelling can further complicate matters. Accordingly, the macromers useful in the present invention have a size sufficiently large such that no substantial amount of macromer penetrates the article to be treated (e.g., a contact lens) causing substantial distortion of the article.
With respect to optical lenses, the macromers preferably have a size sufficiently large to produce a contact lens having a final diameter which differs less than 10% with respect to the diameter of the untreated lens. More preferably, the lens diameter changes less than about 5%, even more preferably, less than about 2%.
One method of measuring the relative size of a macromer is molecular weight. The preferred molecular weight of the macromers is a number-average molecular weight greater than about 500. A more preferred number-average molecular weight is above about 1000.
The previous disclosure will enable one having ordinary skill in the art to practice the invention. In order to better enable the reader to understand specific embodiments and the advantages thereof, reference to the following examples is suggested.